The pathogenicity of Giardia lamblia for man was definitively established. 10/10 individuals inoculated with GS/E became infected and 50% became ill. Differences in the pathogenicity of isolates was additionally demonstrated because none of the 5 Isr inoculated volunteers became infected. IgM responses appear to be the most reliable humoral indicator of infection since all infected persons showed significant IgM antibody responses. Giardia antigens were reliably detected in the stools of infected volunteers despite the variation in cyst excretion. Analysis of the surface antigens and E-S products of new Giardia isolates confirms the diverse heterogeniety of Giandia lamblia in nature. The ability of Giardia to persist in culture despite the presence of high concentrations of antibiotics was studied. Persistance was dependent on the concentration of antibotics, duration of exposure, inoculum size and stability of the antibiotic. Ultrasonography reliably detects Symmers' fibrosis. This technique is now being applied in the study of infected populations in the field. An assay to detect circulating GASP antigen in humans was established. Antibody responses to specific Cryptosporidium antigens was studied using Western blots. Most infected persons recognized a 23kd antigen.